Asphalt is one of the oldest building materials known and is widely used as a paving and a roofing material. Presently, approximately 70% of all asphalts in the United States are consumed by the road paving industry, about 20% are consumed by the roofing industry, and the remainder are used for certain specialty products such as adhesives and coatings. Asphalt occurs in natural deposits and as a by-product from the processing of crude oil. The term "asphalt" is commonly applied today to petroleum residues that, for the most part, are soluble (nearly 100%) in carbon disulfide or aromatics solvents such as toluene.
The physical characteristics of asphalts vary widely, depending upon their constituents. Colloidal dispersions of asphalts present in crude oil are high molecular weight hydrocarbons and are generally referred to as asphaltenes. Asphalts also include oils in which the asphaltenes may be dispersed and resins. At least some of the resins can cause peptization of the asphaltenes. The relative quantities and physical properties of the asphaltenes, the oils, and the resins determine the physical characteristics of a given asphalt. For example, these characteristics are dependent upon the particle size of the asphaltenes as dispersed in the oily phase. A portion of the asphaltenes may be of such a small size as to provide a colloid of the sol type which is in reality a true solution rather than a dispersion. One characteristic of a sol type asphalt is a sharp transition upon heating from a glassy solid at low temperatures to a liquid of relatively low viscosity at elevated temperatures.
The solubility of water in asphalt is quite low and appears to dissolve in asphalt only as a gas. However, greater quantities of water may be absorbed in asphalt compositions because of constituents other than the asphalts, such as filler materials. Inorganic fillers may absorb water to varying degrees, and the presence of water on filler materials may lead to poor adhesion of the asphalt binder to the filler materials.
When used in certain applications such as paving materials, asphalts are mixed with coarse aggregate such as crushed stone, fine aggregate such as sand and stone dust or other inorganic fillers. A typical asphalt paving composition may contain about 54% coarse aggregate, about 35% fine aggregate, about 5% stone dust, and about 6% asphalt which serves as the binder. Paving compositions are generally made up and applied in either one of two ways, namely, cold-mixed and cold-laid or hot-mixed and hot-laid.
Since asphalt serves as the binder in paving and roofing compositions containing a filler material, it is the component that furnishes the adhesion to hold the particles of inorganic mineral aggregates or other filler materials together. Asphalt also furnishes the flow properties necessary to properly wet and coat surfaces of the aggregates and fillers and to apply the resulting mix to a surface or substrate, and the durability of the product as required for a suitable service life. Particularly important properties for a durable and adhesive asphalt are (1) significant plastic deformation without rupture (ductility) in order to resist thermal stresses and load stresses, and (2) significant elastic deformation (elasticity) to permit elastic recovery instead of plastic flow under shock loading so as to absorb shock loads without significant plastic deformation. Nearly all jurisdictions responsible for paving specifications require a ductility test as specified by the Federal Bureau of Public Roads. This ductility test uses a test sample conforming to ASTM Standard D113 and tested in a water bath maintained at 77.degree. F. In order to pass this test, the test sample must have a ductility of at least 50, 75 or 100 centimeters, depending upon the grade of asphalt cement selected, when tested at a rate of 5 centimeters per minute.
Under the influence of atmospheric moisture and ambient temperature variations, as well as load stresses, the asphalt binder may peel off of the aggregate or other filler material after a year or two of use. It is therefore desirable to provide asphalt compositions which are capable of producing pavements, roofing materials and other products having good stripping-resistance in the presence of moisture and varying temperatures. The term "stripping-resistance" as used in this specification means the resistance of the asphaltic binder to peeling from the aggregate or other filler when the mix of asphaltic binder and aggregate has been applied as a film to a substrate, for example, pavement applied to a foundation layer or directly onto soil.
Asphalts applied as paving or roofing essentially function as thin films and these films age and undergo structural changes much more rapidly than when the asphalt is in its bulk form. Aging results in increased hardness of the asphalts due to exposure to various physical agents, such as physical and thermal stresses, and chemical agents, such as water, oxygen, ozone and the ultraviolet band of sunlight. Such hardening occurs at different rates depending on a variety of factors, including the film's permeability to air and water, the range of ambient temperatures to which it is exposed, and the frequency of wetting and drying cycles. Aging also results in increased viscosity and other rheological changes, all of which may lead to relatively early failure in service, i.e., a shortening of the service life of the product. The presence of water also may result in a loss of adhesion between the asphalt and the fillers.
The hardening of asphalts may be due in part to changes in its chemical composition because of a loss of volatiles by evaporation and/or a loss of the oily phase by selective adsorption of oils into the aggregate or by a mechanism referred to as "sweating". When these changes occur, the hardness of the asphalt increases, its ductility decreases, and it becomes even more susceptible to the adverse effects of oxidation and water absorption.
The durability of the asphalt, therefore, is a prime consideration in selecting an asphalt composition for many applications. A number of methods have been suggested in the past in an effort to improve asphalt durability. One method suggested is to blend different asphaltic components separated from naturally occurring and/or petroleum derived asphalts. A second method is the addition of synthetic additives to modify the rheological and chemical properties of an asphalt. Such additives in the past have included various polymers which may lend their rheological properties to those of the asphalt to which they are added.
A rather significant problem associated with the use of prior art polymers is the difficulty of bringing them into solution with the hot asphalt. The time and temperatures required to disperse at least some of these polymers is so high as to result in significant oxidation of the polymer and/or the asphalt. Such oxidation increases the hardness and decreases the ductility of the polymer/asphalt blend. Also, blending these polymers into the asphalt requires high energy mixing over relatively long periods of time (an hour or more). Thus, substantial amounts of money could be saved if the time and energy involved in achieving a homogeneous mixture of the added polymer and the asphalt could be reduced significantly.
In the case of paving applications, it is often necessary to maintain the asphalt-containing material in its heated fluid state for extended periods of time (more than one day). In the presence of the fluidizing heat and high shear mixing conditions required to provide a dispersion of prior art polymers, a serious problem was found to occur wherein certain polymer/asphalt blends undergo a sudden rapid increase in viscosity which may render the blend completely useless or may greatly increase the difficulty of handling the blend for further use. Sometimes it is not possible to identify these blends in advance because only certain asphalts appear to be susceptible to this problem of sudden viscosity increase after an extended mixing time with the polymer.
The loss of ductility of an asphalt with mixing or the onset of a rapid viscosity increase after an extended mixing time for an asphalt/polymer blend may be due, at least in part, to the inclusion of air into the composition by the mixing operation. Extended heating of a modified asphalt composition also may result in depolymerization of the additive polymer and at the same time may cause asphaltenes to coagulate. Another problem with modified asphalts of the prior art is that mixtures containing more than 5% polymer may exhibit a phase reversion wherein the polymer swells and becomes the continuous phase of a colloidal system in which the asphalt is the dispersed phase. This reversion also causes coagulation of the asphaltenes.